Shallow-marine tidal deposits

Abstract

Shallow-marine tidal deposits form on open shelves, and more specifically in open-mouthed embayments and semi-enclosed epicontinental seas, where the oceanic tide is amplified by resonance. They are also present in straits and seaways where the tidal currents are accelerated by flow constriction. Complex interactions of the tide with the seafloor and coastal topography bring about tidal asymmetry, generating tidal-transport pathways with net, unidirectional transport of sediment over long distances. Tidal currents are commonly capable of resuspending mud in shallow-marine settings, but little is known about the role of tidal currents in the deposition of muddy deposits in the offshore domain. The best-known shelf tidal deposits are sandy and bioclastic transgressive 'lags' that mantle flooding surfaces. These lags are generally thin, but can reach thicknesses of 10-30 m in tidal-current ridges and sand sheets. These deposits are composed of dominantly well-sorted, cross-bedded sands with good reservoir properties. Careful architectural analysis allows the distinction between the deposits of compound dunes, tidal-current ridges and migrating sand sheets. The occurrence of shallow-marine tidal deposits is sensitive to changes in sea level; paleotidal modeling has great potential to help understanding their occurrence in space and time.